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5V from the computer’s USB port is used to charge high voltage capacitors through a DC-DC converter up to a predetermined value, something in the order of 100V or higher.

Once the caps are charged, a switch then dumps the stored charge back through the USB’s signal lines and the cycle repeats until 5V is no longer available through the computer’s USB port, indicating the host computer has been damaged - dumping the high voltage back into the 5V line will damage the killer device itself; not a very smart thing to do, hence the signal lines.

How much damage it will do to a computer will be determined by how protected its USB port is - in some cases the motherboard traces will act as a fuse, or the USB controller will fry, and in severe cases the motherboard will be damaged.

A few clamping diodes will actually be adequate to protect the system, but proper ESD protection takes space that’s simply not available in modern slim-and-small devices.